In a conventional electrophotographic copying machine, a photoconductor is charged and subjected to imagewise exposure, through an optical system, to form a latent image of the original information, which may be any document such as a printed page, a drawing, or a photograph. The latent image thus formed is then developed and the developed image transferred to a carrier sheet such as paper. This is the copying mode.
It is also known that electrophotographic apparatus may be operated in a printing mode, in which a modulated scanning laser beam discharges a charged photoconductor as a function of the desired information, thus producing a latent electrostatic image corresponding to the information. This image is then toned and transferred, as in the case of an image formed optically from an original.
It is desirable to provide apparatus which can operate in a scanning mode, as well as the copying mode and printing mode. Optical systems are expensive. Solid-state lasers, as a source of light, are an economical and efficient way to scan an original. Unfortunately, a laser diode, while inexpensive, operates in a very narrow wavelength and is unsatisfactory for scanning a colored original, though it can be used to scan a black and white document. The use of multi-wavelength lasers to scan an original document would be expensive, cumbersome, and unreliable. Thus, the production of apparatus which is capable of operating in the printing mode and the scanning mode or in the copying mode and the scanning mode has not been feasible.
In view of the apparent difficulty of scanning an original document with a laser to derive an input for a facsimile system, electrophotographic devices of the art have produced a latent image by optical means and then scanned that latent image either with a laser beam or with a pin tube. The scanning of the latent image with a laser erases it, as does the scanning of the latent image with an electron beam from a pin tube. It will be seen that the devices of the prior art which operate in the scanning mode can operate in that mode alone. No device of the prior art can operate in both the copying mode and scanning mode and in the printing mode and scanning mode.